1970 mercury 650 runs good uses too much fuel while in gear idling

c1nestor

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Feb 25, 2013
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I have a 1970 Mercury 650 65 horse 4cylinder on a 1970 15' Las Vegas tri hull. I bought the boat about a month ago and love it. I got it to use mainly for trolling. The motor starts and runs well at idle and full throttle. I put it in forward and let it run in idle to troll. I have taken it out twice so far and the first time I almost ran out of gas by noon with about 5 gallons of gas in the tank. Thats about a gallon an hour idling. When I got home I turned the idle mixture screws in about a 1/4 turn to see if it was just running too rich. I took the boat out yesterday and it didn't want to start as easily or run in gear at idle so I returned the idle screws to about the original position. My understanding is that it should be using approximately .3 gallons per hour which seems way more reasonable. Any opinions at all would be greatly appreciated.
 

RogersJetboat454

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

Welcome to Iboats.

I think your expectations are a bit off...
Your motor is a thirsty old 2 stroke, built in 1970 when fuel was around .30? a gallon.
If it starts right up, doesn't foul plugs easily, and runs great in all throttle positions, I don't think you can ask for much more.

I assume you are running it on a single portable fuel tank. Might be wise of you to invest in a second tank as a reserve. ;)
 

c1nestor

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

Thank you for the welcome and quick response. I am relieved to hear that there is no real problem with my motor. It used about 8 gallons of fuel in 9 hours yesterday. I was thinking that it should have used about half that.
 

RogersJetboat454

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

Thank you for the welcome and quick response. I am relieved to hear that there is no real problem with my motor. It used about 8 gallons of fuel in 9 hours yesterday. I was thinking that it should have used about half that.

Yeah..
I'd say you did pretty good for your self.
There's guys around here with bigger motors that will use almost double that in an hours worth of cruising.

Boat's don't coast like anything on land. The motor is constantly under load fighting the resistance of the water. More resistance = More fuel. Plus, like I mentioned, a 2 stroke from 1970 isn't going to be a fuel sipper anyway.
 

c1nestor

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

My next little problem would be with the tachometer. Most of the time it does nothing. Sometimes when I go to full throttle it will come alive and work properly throughout the full rpm range then go dead again. It's a model 666a Airguide Sea speed. I re soldered the corroded connections under the dash and the connections on the switch panel in the motor look good.
 

mercurymang

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

My next little problem would be with the tachometer. Most of the time it does nothing. Sometimes when I go to full throttle it will come alive and work properly throughout the full rpm range then go dead again. It's a model 666a Airguide Sea speed. I re soldered the corroded connections under the dash and the connections on the switch panel in the motor look good.

Does the tach wire come in through the harness or is it wired seperately. Either way, all you can do is make sure you have good wiring all the way through the circuit. If that does not fix it, it may be tach itself.
 

c1nestor

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

It goes through the harness.
 

mercurymang

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Re: 1970 mercury 650 runs good uses too much fuel while in gear idling

Boat wiring gets pretty nasty over time. When I bought mine, the first thing I did was rewire the entire thing. I'm not saying you need to do that but I would redo all the connections in that circuit. If that doesn't fix it, it could be your tach.
 
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